Amplifier circuit, an optical communication system and a method of controlling amplification

ABSTRACT

The invention concerns an amplifier circuit for receiving an optical signal. The circuit comprises a light sensitive member ( 301 ) which delivers an electric signal in response to an optical signal. Furthermore, the amplifier circuit comprises an amplifier unit ( 302 ) with a first input ( 304 ) which receives an electric signal from the light sensitive member ( 301 ). The amplifier unit ( 302 ) is of the kind which does not have any special input intended for controlling the amplification. A filter unit ( 310 ) prevents a possible direct current from the light sensitive member ( 301 ) from reaching the first input ( 304 ). A control unit ( 314 ) is connected to the first input ( 304 ) for controlling the amplification of the circuit. The invention also concerns an optical communication system comprising an amplifier circuit of the above-described kind. The invention also concerns a method of controlling the amplification in an optical input stage.

BACKGROUND OF THE INVENTION AND PRIOR ART

[0001] The present invention concerns an amplifier circuit for receivingan optical signal, which amplifier circuit comprises:

[0002] a light sensitive member arranged to receive an optical inputsignal and to deliver an electric signal in response to the receivedoptical signal, and

[0003] an amplifier unit with a first input arranged to receive theelectric signal from the light sensitive member, wherein the amplifierunit is arranged to influence the amplification of the electric signaland to deliver an amplified output signal via a first output, whereinthe amplifier unit does not have any further input specially intendedfor controlling the amplification of the amplifier unit.

[0004] The invention also concerns a communication system and a methodof controlling the amplification in an optical input stage.

[0005] The above described amplifier circuit may suitably be used foramplifying an input signal originating from an optical fibre. Theamplifier circuit may thus constitute a fibre optic input stage. Such aninput stage is used in optical communication systems. Such an inputstage may be followed by further amplifiers. The input stage thus oftenensures that an input signal from an optical communication path isadapted to an input of a subsequent amplifier.

[0006]FIG. 1 shows schematically an example of such an optical inputstage according to the prior art. The input stage comprises a lightsensitive member 301. The light sensitive member 301 may for exampleconstitute a photo-diode. The light sensitive member 301 delivers anelectric signal in response to an optical input signal, for example froman optical fibre (not shown in the figure). According to the shownexample, the cathode of the photo-diode 301 is connected to a biasvoltage V1. The circuit comprises an amplifier unit 302, which is oftencalled preamplifier. The amplifier unit 302 has a first input 304 whichreceives an electric signal from the light sensitive member 301. Theamplifier unit 302 influences the amplification of the electric signaland delivers an amplified output signal via a first output 306. Theshown photo-diode 301 delivers a current into the input 304, wherein thestrength of the current depends on detected light. The amplifier unit302 converts the current to a voltage at the output 306. The transferfunction therefore gets the unit V/A, i.e. ohm. The amplification of theamplifier unit 302 may thus be stated in ohm. The amplifier unit 302 mayalso comprise an internal amplification control unit 308 which forexample may be arranged to reduce the amplification at too highcurrents.

[0007] The English abstract of JP-A-10284955 shows an example of thiskind of optical input stage. This document shows such an input stagewith a control circuit for controlling the amplification in response toan average value of the optical input power.

[0008] Also WO99/28768 shows an optical input stage where a controlcircuit controls a variable impedance element in the form of a diodeconnected to an amplifier input.

[0009] The English abstract of JP-A-09298426 shows an optical inputstage with a preamplifier. In this case the preamplifier has a specialinput where a control signal may be connected for controlling theamplification of the preamplifier. However, an optical input stageusually lacks a special input for controlling the amplification. Anexample of an input stage is the one which is sold with the name MC2006of the fabrication Microcosm. For example this input stage comprises aninternal control unit for reducing the amplification at too highcurrents. However, the input stage lacks a special input for being ableto control the amplification.

SUMMARY OF THE INVENTION

[0010] A purpose of the present invention is to achieve an amplifiercircuit which makes it possible to control the amplification in anoptical input stage with an amplifier unit which does not have anyspecial input intended for controlling the amplification. Anotherpurpose is to achieve such an amplifier circuit by relatively simplemeans.

[0011] These purposes are achieved by an amplifier circuit according tothe first paragraph above and which is characterised by a filter unitarranged to prevent a possible direct current in the electric signalfrom the light sensitive member from reaching said first input, and acontrol unit connected to said first input and arranged to control thepower of the electric signal at this first input for thereby influencingthe power of said output signal.

[0012] The filter unit may for example constitute a capacitor. Thisfilter unit disconnects a possible direct current from the lightsensitive member. Instead the control unit is connected to the firstinput. The power of the input signal at the first input is thuscontrolled with the help of the control unit instead of with the help ofa direct current from the light sensitive member. Thereby, the power ofthe output signal from the second output may be influenced with the helpof the control unit, i.e. the purpose to be able to control theamplification of the amplifier unit is achieved.

[0013] According to an embodiment of the invention, said light sensitivemember is arranged such that a possible direct current from the lightsensitive member would be directed into said amplifier unit via thefirst input if said filter unit were not arranged to prevent such adirect current, wherein the control unit is arranged to control thepower of an electric current in the direction in via the first input.According to this embodiment, a light sensitive member may thus be usedwhere the current into the first input depends on detected lightintensity.

[0014] According to a further embodiment, the control unit is arrangedas a controllable current generator in that a variable amplificationcontrolling voltage unit is connected to a first resistance connected tosaid first input. This embodiment constitutes an advantageous and simpleexample of how the current into the first input may be controlled.

[0015] According to still an embodiment, said light sensitive membercomprises a first diode-unit with a first connection connected to a biasvoltage and a second connection connected to said filter unit, whichfilter unit is connected to the first input. According to thisembodiment, for example a photo-diode of the kind which has beendescribed above may thus be used as light sensitive member.

[0016] According to still an embodiment, the first connectionconstitutes the cathode side of the first diode unit and the secondconnection constitutes the anode side of the first diode unit. Aphoto-diode usually functions such that its reverse current depends onincident light intensity. According to this embodiment, the current intowards the first input is thus controlled with the help of the diodeunit.

[0017] According to a further embodiment, the amplifier circuitcomprises a second resistance connected to said second connection forconducting away a possible direct current through the first diode unit.Since a direct current from the light sensitive member is prevented fromreaching the first input, this current must be conducted away in asuitable manner. A simple construction is to connect the light sensitivemember to a resistance which conducts this current away.

[0018] According to still an embodiment, the amplifier circuit comprisesan amplification limiting unit connected to said first input. Hereby isprevented that the amplifier unit is set at a too high amplification.

[0019] According to another embodiment, the amplification limiting unitcomprises a third resistance connected between a voltage source and saidfirst input. If the amplifier unit is of the kind where a lower currentat the first input means a higher amplification, then suitably with thehelp of such a third resistance it may be ensured that always a certaincurrent is present at said first input. Thus a limitation of theamplification has been achieved in a simple manner.

[0020] According to another embodiment, said amplifier unit comprises aninternal amplification controlling unit, for example arranged forreducing the amplification at too high currents. As has been mentionedabove, the amplifier unit may also have an internal amplificationcontrol.

[0021] According to a further embodiment, a second diode unit isconnected to said control unit for preventing incorrect currentdirection to/from the first input. For example if the amplifier circuitis arranged such that always a certain current is input via said firstinput, then the second diode unit may be arranged for preventing acurrent in the opposite direction.

[0022] According to still an embodiment, the amplifier circuit comprisesa filter member connected to said control unit for filtering outpossible disturbances in the electric signal which is controlled by thecontrol unit. It is hereby ensured that possible disturbances do notreach the first input.

[0023] According to a further embodiment, the amplifier circuitcomprises a fourth resistance with a first side connected to said lightsensitive member and a second side arranged to be connectable to ameasuring unit for measuring the current through the light sensitivemember. It may be desirable to be able to measure the photocurrentthrough the light sensitive member, for example for determining theoptical input power. In order to prevent that the measuring unit isconnected to a point which is sensitive to disturbances, suitably such afourth resistance may be arranged.

[0024] Another purpose of the invention is to achieve an opticalcommunication system. This purpose is achieved by an opticalcommunication system comprising a first-transmitter-receiver unit, asecond transmitter-receiver unit and at least one optical conductionmember which connects the first and the second transmitter-receiverunits, wherein at least one of said transmitter-receiver units comprisesan amplifier circuit according to any of the preceding embodiments. Suchan optical communication system has the advantage that the amplificationof the input stage may be controlled according to the above-describedembodiments.

[0025] As has been mentioned above, another purpose of the invention isto offer a method for controlling the amplification in an optical inputstage which comprises a light sensitive member arranged to receive anoptical input signal and to deliver an electric signal in response tothe received optical signal, and an amplifier unit with a first inputarranged to receive the electric signal from the light sensitive member,wherein the amplifier unit is arranged to influence the amplification ofthe electric signal and to deliver an amplified output signal via afirst output, wherein the amplifier unit does not have any further inputspecially intended for controlling the amplification of the amplifierunit.

[0026] This purpose is achieved by the following method steps: that afilter unit is arranged to prevent a possible direct current in theelectric signal from the light sensitive member from reaching said firstinput, and that a control unit is connected to said first input andarranged to control the power of the electric signal at this first inputfor thereby influencing the power of said output signal. This method hasthe same advantages as have been described above in connection with theamplifier circuit according to the invention.

[0027] Advantageous embodiments of the method are clear from the otherdependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The present invention will now be explained with the help ofembodiments and with reference to the annexed drawings.

[0029]FIG. 1 shows schematically an optical input stage according to theprior art.

[0030]FIG. 2 shows schematically a simple embodiment of the presentinvention.

[0031]FIG. 3 shows another embodiment of the present invention.

[0032]FIG. 4 shows schematically an optical communication systemaccording to the invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0033]FIG. 2 shows an amplifier circuit according to the invention. Theamplifier circuit comprises a light sensitive member 301, for example aphoto-diode. In the shown example, the cathode of the photo-diode isconnected to a bias voltage V1. The anode of the photo-diode 301 isconnected to a filter unit 310, in this case a capacitor, which in itsturn is connected to a first input 304 of the amplifier unit 302. Thecapacitor 310 prevents a direct current from the photo-diode 301 fromreaching the first input 304. The hatched line 312 symbolises that sucha direct current is conducted away from the photo-diode 301. Theamplifier unit 302 has a first output 306 where an amplified outputsignal is delivered. The amplifier unit 302 is of the kind which doesnot have any special input intended for controlling the amplification ofthe amplifier unit 302. Such an amplifier unit 302 may suitablyconstitute a preamplifier component of the kind which has been describedinitially above. Such a component 302 may comprise an internalamplification controlling unit 308. Such an amplification controllingunit 308 may, but does not have to, comprise a feedback control loop.

[0034] According to the invention, a control unit 314 is connected tothe first input. According to the shown example, the control unit 314constitutes a variable current generator. With this current generator314 the current into the first input 304 may be controlled. The currentgenerator 314 may thus be used for influencing the amplification of thecircuit.

[0035] It should be noted that the figures only show preferredembodiments. Within the scope of the invention is of course thepossibility that for example the polarity of the circuit may be theopposite. With reference to FIG. 2, for example the photo-diode 301could be reversed and V1 could be a negative voltage. The control unit314 would in this case control a current out from the first input 304.

[0036]FIG. 3 shows a further embodiment of the invention. Thecorresponding parts as in FIG. 2 have the same reference signs as inFIG. 2. These parts will therefore not be described more closely inconnection with FIG. 3. According to FIG. 3, the control unitconstitutes a variable amplification-controlling voltage unit 316connected to a first resistance 318 which in its turn is connected tothe first input 304. A second diode unit 320 is arranged for preventingan incorrect current direction, i.e. in this case a current out from thefirst input 304. A filter member 322, in this case a capacitor, isarranged to filter out possible disturbances which are superposed on theamplification-controlling voltage.

[0037] A second resistance 324 is connected to the anode of thephotodiode 301 for conducting away a direct current. According to theshown embodiment, the amplifier unit 302 is of the kind where a lowcurrent into the first input 304 means a high amplification. In order toensure that a certain current is always present at the first input 304,a third resistance 326 is arranged between the bias voltage V1 and thefirst input 304. This third resistance 326 thus limits the amplificationof the circuit.

[0038] As has been mentioned above, it may be interesting to measure thephoto current through the photo-diode 301. The photo current isproportional to the voltage over the second resistance 324. However, itmay be unsuitable to measure this voltage since such a measurement couldlead to disturbances in a sensitive part of the circuit. In order toavoid this problem, a fourth resistance 330 is connected to the anode ofthe photo-diode 301. This fourth resistance is suitably a resistancewith a high resistance. A measurement device 332 may thus be connectedto this fourth resistance 330.

[0039]334 symbolises a circuit which follows after the amplifier unit302. This circuit 334 may comprise an amplifier. The hatched line 336symbolises that a feedback from this circuit 334 may be arranged forinfluencing the variable voltage which is symbolised by the unit 316.

[0040]FIG. 4 shows schematically an optical communication systemaccording to the invention. Such a communication system comprises afirst transmitter-receiver unit 340. This unit comprises a transmitter342 and a receiver 344. A second transmitter-receiver unit is designatedby 346. This second transmitter-receiver unit 346 comprises a receiver348 and a transmitter 350. The communication system comprises a first352 and a second 354 optical fibre. According to the invention, at leastone of said transmitter-receiver units 340, 346 comprises an amplifiercircuit according to the invention. Suitably both transmitter-receiverunits 340, 346 comprises such an amplifier circuit.

[0041] The invention also concerns a method of controlling theamplification in an optical input stage of the kind which has beendescribed above, i.e. an amplifier circuit which comprises a lightsensitive member 301 arranged to receive an optical input signal and todeliver an electric signal in response to the received optical signal,and an amplifier unit 302 with a first input 304 arranged to receive theelectric signal from the light sensitive member 301, wherein theamplifier unit 302 is arranged to influence the amplification of theelectric signal and to deliver an amplified output signal. The amplifierunit 302 is of the kind which does not have any further input speciallyintended for controlling the amplification of the amplifier unit.According to the method, a filter unit 310 is arranged for preventing apossible direct current in an electric signal from the light sensitivemember 301 from reaching the first input 304 of the amplifier unit 302.Furthermore, a control unit 314 is connected to the first input 304 forcontrolling the power of the electric signal at this first input 304.Thereby the amplification of the input stage is influenced.

[0042] Advantageous embodiments of the method are clear from the claims15 and 16. Further advantageous embodiments of the method are clear fromthe above-described function of the amplifier circuit.

[0043] The invention is not limited to the described embodiments but maybe varied and modified within the scope of the following claims.

1. An amplifier circuit for receiving an optical signal, which amplifiercircuit comprises: a light sensitive member (301) arranged to receive anoptical input signal and to deliver an electric signal in response tothe received optical signal, an amplifier unit (302) with a first input(304) arranged to receive the electric signal from the light sensitivemember (301), wherein the amplifier unit (302) is arranged to influencethe amplification of the electric signal and to deliver an amplifiedoutput signal via a first output (306), wherein the amplifier unit (302)does not have any further input specially intended for controlling theamplification of the amplifier unit, characterised by a filter unit(310) arranged to prevent a possible direct current in the electricsignal from the light sensitive member (301) from reaching said firstinput (304), and a control unit (314) connected to said first input(304) and arranged to control the power of the electric signal at thisfirst input (304) for thereby influencing the power of said outputsignal.
 2. An amplifier circuit according to claim 1, wherein said lightsensitive member (301) is arranged such that a possible direct currentfrom the light sensitive member (301) would be directed into saidamplifier unit (302) via the first input (304) if said filter unit (310)were not arranged to prevent such a direct current, wherein the controlunit (314) is arranged to control the power of an electric current in adirection in via the first input (304).
 3. An amplifier circuitaccording to claim 1 or 2, wherein the control unit (314) is arranged asa controllable current generator in that a variableamplification-controlling voltage unit (316) is connected to a firstresistance (318) connected to said first input (304).
 4. An amplifiercircuit according to any of the preceding claims, wherein said lightsensitive member (301) comprises a first diode unit (301) with a firstconnection connected to a bias voltage (V1) and a second connectionconnected to said filter unit (310), which filter unit (310) isconnected to the first input (304).
 5. An amplifier circuit according toclaim 4, wherein the first connection constitutes the cathode side ofthe first diode unit (301) and the second connection constitutes theanode side of the first diode unit (301).
 6. An amplifier circuitaccording to any of the claims 4 and 5, comprising a second resistance(324) connected to said second connection for conducting away a possibledirect current through the first diode unit (301).
 7. An amplifiercircuit according to any of the preceding claims, comprising anamplification limiting unit (326) connected to said first input (304).8. An amplifier circuit according to claim 7, wherein the amplificationlimiting unit (326) comprises a third resistance (326) connected betweena voltage source (V1) and said first input (304).
 9. An amplifiercircuit according to any of the preceding claims, wherein said amplifierunit (302) comprises an internal amplification controlling unit (308),for example arranged for reducing the amplification at too highcurrents.
 10. An amplifier circuit according to any of the precedingclaims, comprising a second diode unit (320) connected to said controlunit (314) for preventing incorrect current direction to/from the firstinput (304).
 11. An amplifier circuit according to any of the precedingclaims, comprising a filter member (322) connected to said control unit(314) for filtering out possible disturbances of the electric signalwhich is controlled with the control unit (314).
 12. An amplifiercircuit according to any of the preceding claims, comprising a fourthresistance (330) with a first side connected to said light sensitivemember (301) and a second side arranged to be connectable to a measuringunit (332) for measuring the current through the light sensitive member(301).
 13. An optical communication system comprising a firsttransmitter-receiver unit (340), a second transmitter-receiver unit(346) and at least one optical conduction member (352, 354) whichconnects the first (340) and the second (346) transmitter-receiverunits, wherein at least one of said transmitter-receiver units (340,346) comprises an amplifier circuit according to any of the precedingclaims.
 14. A method for controlling the amplification in an opticalinput stage which comprises a light sensitive member (301) arranged toreceive an optical input signal and to deliver an electric signal inresponse to the received optical signal, an amplifier unit (302) with afirst input (304) arranged to receive the electric signal from the lightsensitive member (301), wherein the amplifier unit (302) is arranged toinfluence the amplification of the electric signal and to deliver anamplified output signal via a first output (306), wherein the amplifierunit (302) does not have any further input specially intended forcontrolling the amplification of the amplifier unit (302), which methodcomprises the following steps: that a filter unit (310) is arranged toprevent a possible direct current in the electric signal from the lightsensitive member (301) from reaching said first input (304), and that acontrol unit (314) is connected to said first input (304) and arrangedto control the power of the electric signal at this first input (304)for thereby influencing the power of said output signal.
 15. A methodaccording to claim 14, wherein said light sensitive member (301) isarranged such that a possible direct current from the light sensitivemember (301) would be directed into said amplifier unit (302) via thefirst input (304) if said filter unit (310) were not arranged to preventsuch a direct current, wherein the control unit (314) is arranged tocontrol the power of an electric current in a direction in via the firstinput (304).
 16. A method according to claim 14 or 15, wherein thecontrol unit (314) is arranged as a controllable current generator inthat a variable amplification controlling voltage (316) is applied overa first resistance (318) connected to said first input (304).